Synthesis of glasses from gels: the problem of monolithic gels

Zarzycki, J.; Prassas, M.; Phalippou, J.

doi:10.1007/bf01203507

Cited by 333 publications

(97 citation statements)

References 18 publications

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“…A systematic investigation design to optimize supercritical extraction conditions to produce monolithic aerogels was carried out at Montpellier. [26][27][28][29] Two different strategies can be used to bypass the critical point: (1) adding an extra volume of liquid in the autoclave and heating; (2) applying pressure using an inert gas before heating. 30 The resulting product is a gel with its pores filled with air, the source of the term aerogel.…”

Section: An Insight Into the Sol±gel Processmentioning

confidence: 99%

Sonogels and derived materials

Blanco¹,

Esquivias²,

Litrán³

et al. 1999

Appl. Organometal. Chem.

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Nowadays the sol–gel process is used by an increasing number of researchers for the preparation of various products, including bulk materials, films, membranes or fibers. The application of ultrasound (sonocatalysis) to the precursors gives rise to materials with new properties, known as sonogels. The absence of additional solvent and, mainly, the effects of ultrasonic cavitation create a unique environment for sol–gel reactions leading to particular features in the resulting gels: high density, fine texture, homogeneous structure etc. These properties determine the evolution of sonogels on further processing and the final material structure. In this sense, the full exploitation of sonocatalysis requires a thorough understanding of the processes involved and their sensitivity to reaction parameters. For this purpose, we have used diverse techniques to investigate the microstructural evolution during different steps in the sonogel process. The results of these studies are reported here. Finally, we present an overview of some applications for which the mechanical, textural and optical characteristics of sonogels are quite useful. Copyright © 1999 John Wiley & Sons, Ltd.

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Section: An Insight Into the Sol±gel Processmentioning

confidence: 99%

Sonogels and derived materials

Blanco¹,

Esquivias²,

Litrán³

et al. 1999

Appl. Organometal. Chem.

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“…To avoid cracking the drying rate must be extremely slow; days, and even months, may be needed to dry a gel 1cm thick without cracking; the larger the gel, the longer the time required. This problem has been discussed by Iler53) and by Zarzycki et al 114 Nogami and Moriya86) showed that silica gels produced using ammonium hydroxide as a catalyst have a granular appearance in the TEM, but the structure of an acid-catalyzed gel was too fine to see. Nitrogen adsorption analysis of the latter indicated a bimodal pore size distribution, consisting of 2 and 7nm pore diameters.…”

Section: Dryingmentioning

confidence: 89%

Structural Evolution of Sol-Gel Glasses

Scherer

1987

Journal of the Ceramic Association, Japan

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This review examines the stages of the sol-gel process, including hydrolysis, condensation, gelation , aging, drying, and sintering. The species produced in the sol are polymeric, rather than being dense glass-like colloidal particles . Considerable control over the structure of the polymer is possible through an understanding of the chemistry of hydrolysis and condensation . The properties of the gel and its response to heat treatment are sensitive to the structure created in the sol stage. Solvent must be removed slowly to prevent the high capillary stresses from causing cracking. A model of drying is presented that explains the relationship between cracking, drying rate, gel size, and permeability. Heat treatment causes densif ication of the solid phase , as well as collapse of the pores. The sintering behavior of gels is complex, because the viscosity of the gel is affected by concurrent structural relaxation and changes in hydroxyl content.

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“…Aging a gel before drying helps to strengthen the network and thereby reduces the risk of fracture [29]. The effects of the gel aging period on the volume shrinkage (%) and density of aerogels were studied varying the aging period from 0 to 4 h (table 2) by keeping the tartaric acid/Na 2 SiO 3 molar ratio constant at 1.08.…”

Section: Effect Of Gel Aging Periodmentioning

confidence: 99%

A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

Bangi

Rao

2008

Science and Technology of Advanced Materials

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An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol-gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na 2 SiO 3 , gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435• C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na 2 SiO 3 : H 2 O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane-or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m −3 density, 0.090 W mK −1 thermal conductivity, 95% porosity and a contact angle of 146• with water.

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Synthesis of glasses from gels: the problem of monolithic gels

Cited by 333 publications

References 18 publications

Sonogels and derived materials

Sonogels and derived materials

Structural Evolution of Sol-Gel Glasses

A new route for preparation of sodium-silicate-based hydrophobic silica aerogels via ambient-pressure drying

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